The present invention relates to a method of controlling a printed object.
In particular, the present invention relates to a method of controlling banknotes, to which the following description refers purely by way of example.
As is known, banknotes are made from a sheet of such a size as to contain several finished banknotes, and which comprises watermarked regions and/or regions including metal bands, and is fed along a given path along which it undergoes various printing steps, each involving a different printing technique.
The most common printing steps are offset printing, i.e. of images normally representing a coloured background; intaglio or copperplate printing, i.e. of famous figures and/or faces and numbers representing the value of the banknote; and silk-screen printing, i.e. of the serial number. Offset printing is performed on both faces with no alignment with the edge of the sheet, which therefore cannot be used as a reference for the coordinates of the offset printed details.
Intaglio printing is performed at high pressure using a plate, and deforms the paper slightly so that the plate is inclined, and the intaglio print itself possibly misaligned, with respect to the offset print. Intaglio printing is performed on one or both faces of the sheet, and may comprise a number of successive prints, each of which may be misaligned with respect to the offset or other intaglio prints. Misalignment of the various successive prints may take the form of translation along the x and/or y axis, or rotation. Similarly, the silk-screen print may also be translated and inclined with respect to the offset print.
Once offset and intaglio printed, the sheet is quality controlled and cut into separate banknotes. The quality control process consists in assigning a pass or reject signal to each banknote, and only the passed banknotes are silk-screen printed with serial numbers.
Quality control is frequently performed manually by a checker, who ensures the offset and intaglio prints do not deviate too far from an ideal value, and checks for coloring errors, i.e. over- or underinked regions, blurring or other defects.
Alternatively, quality control may also be performed automatically using a television camera, which assigns each pixel a characteristic luminance value of a given banknote surface associated with the pixel, and compares these values with given reference values. Automatic control is complicated by numerous factors, foremost of which is determining reference values enabling accurate control of both coloring and misalignment, which in turn is complicated by the luminance of each pixel depending on various factors, such as the printed region partly occupying the surface corresponding to the pixel, and the type of ink and paper used. The problem is further compounded by misalignments of up to a millimeter between the intaglio and offset prints being considered acceptable, and by effective colour control requiring the use of television cameras of such definition that each pixel corresponds to a banknote surface portion of 0.125xc3x970.125 square millimeters. Misalignment to the above extent, in elation to the size of said surface portion, therefore means the luminance value of each pixel may vary within a very wide range, due to the surface portion corresponding to the pixel possibly being fully inked or having no ink at all, particularly when the surface portion in question is located at the edge of a figure. On the other hand, reference values establishing very wide acceptance ranges would fail to provide for effective control by possibly passing banknotes which should be rejected.
It is an object of the present invention to provide a control method enabling reliable automatic control of both misalignment and coloring.
According to the present invention, there is provided a method of controlling a printed object comprising a first and a second print executed at different stages; the method comprising the steps of acquiring a first image of the controlled object; comparing said first image with a specimen second image; calculating a relative deviation between the first print and said second print of said object; and emitting an error signal in the event the luminance values of the first image fail to fall within an acceptance range of the luminance values of the second image, or said relative deviation is above a given value; said method being characterized in that said second image is a virtual image formed by superimposing a reference third image of the first print and a reference fourth image of the second print, and taking into account said relative deviation.
The method according to the present invention is particularly advantageous by controlling relative deviation of the first and second prints and coloring in two separate steps using a specimen image which takes into account existing relative deviations of the first and second prints, thus enabling precise, reliable acceptance ranges to be established.